Magnet Resonance Imaging (MRI) is a non-invasive method using very strong magnetic fields to render images of the inside of an object and is primarily used in medical imaging to demonstrate pathological or other physiological alterations of living tissues. In addition Positron Emission Tomography (PET) is another medical imaging method, where a short-lived radioactive tracer isotope, which decays by emitting a positron, is injected usually into the blood circulation of a living subject. After the metabolically active molecule becomes concentrated in tissues of interest, the research subject or patient is placed in the imaging scanner. The molecule most commonly used for this purpose is fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour.
As the radioisotope undergoes positron emission decay, it emits a positron, the antimatter counterpart of an electron. After traveling up to a few millimeters the positron encounters and annihilates with an electron, producing a pair of gamma photons moving in almost opposite directions. These are detected in the scanning device by a detector assembly, typically a scintillator material coupled to a photomultiplier, which converts the light burst in the scintillator into an electrical signal. The technique depends on simultaneous or coincident detection of the pair of photons.
Both scanning methods have their particular advantages, thus, diagnosis often requires both scanning methods. The latest complex scanning devices, thus, combine MRI and PET scanner in a way, that both devices can operate in parallel. Traditionally, normalization of a the PET scanner is performed by using an electric motor that rotates a calibration radiation source within a PET gantry. However, if an MRI and a PET scanner are combined, the strong magnetic fields of an MRI make it impossible to operate a normal electric motor. Thus, using traditional normalization methods, the PET insert camera within an MRI system must be normalized outside the MRI.